Communication devices such as wireless devices are also known as, e.g., User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Wireless devices are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system, wireless communications network, or cellular network. The communication may be performed, e.g., between two wireless devices, between a wireless device and a regular telephone, and/or between a wireless device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, tablets or surf plates with wireless capability, just to mention some further examples. The wireless devices in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless device or a server.
The wireless communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g., a Radio Base Station (RBS), which sometimes may be referred to as, e.g., “eNB”, “eNodeB”, “NodeB”, “B node”, or Base Transceiver Station (BTS), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the wireless device. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the wireless device to the base station.
Beamforming is a general set of techniques to control a radiation pattern of a radio signal. To achieve this, several antenna elements may be used to control a total antenna pattern by adjusting transmit weights of signal components radiating from each individual antenna element with purpose of directing the transmitted energy towards a position of an intended receiver.
Beamforming, in general, is an enabler for enhancing the capacity and the energy efficiency in a wireless communications network. The received signal strength is increased due to an increased antenna gain resulting from the beamforming operation. At the same time, interference is spread over a smaller area, typically resulting in reduced interference levels for all user equipments, such as wireless devices, in the system. Increased Signal to Interference plus Noise Ratio (SINR) results in higher bit-rates and higher capacity. Higher SINR in a packet oriented system results in shorter packet transmission times. This also helps to reduce the energy consumption in the system, such as the wireless communications network, since transmitters and receivers may be put into idle mode during a larger ratio of time.
In beamforming, in particular at sub-millimeter Wave (mmW) frequencies, e.g., 10-30 GigaHertz (GH) and mmW frequencies, e.g., 30-300 GH, an antenna, even with many antenna elements, may remain reasonable sized, since the size of each antenna element and distance between antenna elements decreases with increasing frequency. Furthermore, if omni-directional antennas are used at mmW frequencies, the received power decreases, since the effective antenna area of an omni-directional antenna decreases with frequency. To compensate for this effect, the antenna area relative to the area of the omni-directional antenna, thus aperture, may be increased. But the antenna area still remains reasonable sized due to the shorter wave lengths at mmW frequencies, relative to traditional cellular frequencies, to capture more power resulting in directive antennas. Moreover, coverage becomes more challenging at higher frequencies due to increased path loss since propagation mechanisms such as diffraction are frequency dependent, which may also be compensated for with beamforming. Beamforming may be used to implement directive antennas since beamforming enables adjustable beam directions.
FIG. 1 depicts one possible beamforming hardware setup 100. The input to the hardware is typically a baseband signal, e.g. a complex-valued quantity represented by an In-phase (I) and a Quadrature (Q) component. A baseband signal is a signal whose frequency range is a frequency range extending over frequencies close to 0 Hz. It is common to perform parts of the signal processing in baseband and then, usually in an up-conversion mixer in radio, convert it to the desired carrier frequency. The baseband signal may be fed to the beamforming hardware setup 100 in the form or separate streams or layers. In LTE, a layer may be defined by a reference signal. It may be described as an information bearing signal carrying data for a single user. A layer is associated with a continuous or non-continuous Frequency Division Multiplexing/Code Division Multiplexing (FDM/CDM) allocation. A layer may be transmitted using a set of beamforming weights, also known as precoders, on a set of antennas. The layer is composed of component signals. In FIG. 1, three layers or streams, depicted as squares Stream 1, Stream 2 and Stream 3, are fed to the beamforming hardware setup 100 through a BaseBand port (BB herein) 110. The BB 110 may be defined as an interface in a radio Transmitter (Tx), or Receiver (Rx), over which an information carrying signal is conveyed, where the information carrying signal has a center frequency lower than an intended carrier frequency. An Inverse Fast Fourier Transform (IFFT) 112 may be used to transform a signal from frequency domain representation to time domain representation. The BB 110 may be associated with a number of Digital to Analog Converters (DAC) 115 in Tx direction. A number of phase shifters 116 may be comprised in the beamforming hardware setup 100. A number of mixers 117 may also be comprised. The mixers 117 may convert a signal from a lower center frequency, such as zero, i.e., analog baseband, or an intermediate frequency up to a Radio Frequency, RF. In FIG. 1, all BBs 110 share, i.e., reuse, the same set of antenna elements 120. The advantage is that the number of antenna elements 120 may be kept low since the antenna elements 120 are reused across the BBs. The disadvantage is that a Power Amplifier (PA) 130, out of a number of PAs 130, is always fed with a high Peak to Average Power Ratio (PAPR) signal since, even if each baseband signal has low PAPR, its input signal is the superposition of multiple signals, and superposition of signals results in high PAPR. In FIG. 1, the lower arrow indicates that phase shifting, or more generally, applying beam forming weights, may be done at baseband, prior to up-conversion. However, it may also be done at some intermediate frequency or directly at the RF before the PA 130 and before a combiner/summer, upper arrow. The upper arrow indicates combining of the phase shifted baseband signals prior to respective up-conversion and the PA 130. As for the phase shifters, beam forming weights, this may be done at some intermediate frequency or directly at the RF frequency, before the PA 130. This combiner may, however, not be needed if there is one PA per stream and a phase shifted baseband signal. The beamforming hardware setup 100 also comprises a Local Oscillator (LO) 140.
Another hardware setup 200 is depicted in FIG. 2, where each BB is connected to its own set of antenna elements. In the example depicted in FIG. 2 for this hardware setup 200, there are four different BBs: BB1 221, BB2 222, BB3 223 and BB4 224. Thus, in this alternative hardware setup 200, antenna elements are not reused across the BB, but each BB has its own set of antenna elements. The example of FIG. 2 also comprises a radio chain for each of the BB. A radio chain may be described as a group of physical components that are comprised between the DAC 115, which is comprised in the radio chain, and one antenna element or an array of antenna elements connected, through the components, to the DAC 115. The components may enable processing of a signal fed to the DAC 115 for radio transmission of the processed signal through the antenna element, or the array. The one antenna element or the array of antenna elements may also be comprised in the radio chain, as depicted in the example of FIG. 2. In FIG. 2, an exemplary radio chain is indicated with a dashed rectangle. In this example, the radio chain may comprise: the DAC 115, the number of phase shifters 116, the number of mixers 117, the number of PAs 130, the LO 140, and the number of antenna elements 120. Other components, which are not depicted, may also be comprised in the radio chain.
The drawback of a setup such as that of FIG. 2 is the increased number of antenna elements 120, which may lead to increased cost and increased size. On the positive side, the input signal to the PA may have low PAPR, provided the baseband signal has low PAPR, e.g. Discrete Fourier Transform Spread-Orthogonal Frequency Division Multiplexing (DFTS-OFDM). The hardware setup 200 may be used for transmissions with or without using beamforming.
Thus, while the architecture of a system such as that of FIG. 2 provides advantages over that of FIG. 1, it still suffers from high cost and size to the increased number of antennas, which may not be optimally utilized.